During drilling operations for extraction of hydrocarbons, a variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during various phases of drilling rig operations. The use of measurements while drilling (MWD) with real time data transmission provides substantial benefits during a drilling operation. For example, monitoring of downhole conditions allows for an immediate response to potential well control problems and improves mud programs.
Measurement of parameters such as weight on bit, torque, wear and bearing condition in real time provides for more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques.
One current approach to provide real time data from the vicinity of the drill bit to the surface includes acoustic communications. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low intensity of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission. Such reflective and refractive interference causes interbit interference among the bits of data being transmitted.
Acoustic communications are further complicated when surface slips are used on the drill rig floor. For example, surface slips may be used to provide support during periods when drill stand is being added or removed to the drill pipe. In particular, during drilling operations additional drill pipe may be periodically installed to increase the depth of such drill pipe downhole or remove as part of a tripping operation. The surface slips generally surround an opening in the rig floor through which the upper end of the uppermost joint of drill pipe protrudes. The surface slips hold the protruded drill pipe in position (a few feet above the surface of the rig floor) to allow rig personnel and/or automated handling equipment to attach or remove drill pipe. However, the surface slips may act as a reflective point for the acoustic communications between downhole and surface instrumentation, thereby distorting and/or damping such communications.